The present invention relates to a magnetic resonance imaging apparatus, and to a magnetic resonance imaging apparatus which executes an imaging sequence for obtaining magnetic resonance signals generated by transmitting RF pulses to a subject SU in a static magnetic field space and transmitting gradient pulses to the subject to which the RF pulses are transmitted, as imaging data, and thereby generates images about the subject on the basis of the imaging data obtained by the execution of the imaging sequence.
A magnetic resonance imaging (MRI) apparatus is frequently made available for a medical use in particular as an apparatus for photographing an image about a tomographic plane of a subject, using a nuclear magnetic resonance (NMR) phenomenon.
In the magnetic resonance imaging apparatus, a subject is accommodated in an imaging space formed with a static magnetic field thereby to arrange spins of proton in the subject in the direction in which the static magnetic field is formed, and a magnetization vector thereof is produced. A scan for applying an RF pulse having a resonant frequency to generate a nuclear magnetic resonance phenomenon, thereby flipping the spins and, after the magnetization vector of the proton is changed, receiving magnetic resonance (MR) signals produced when the spins are arranged along the static magnetic field direction and the proton is returned to the original state of magnetization vector, is executed as an imaging sequence. The magnetic resonance signals obtained by execution of the imaging sequence are set as imaging data, and images such as a slice image and the like about the subject are generated.
In the present magnetic resonance imaging apparatus, blood photography called “MRA (MR angiography)” is carried out to represent or project flows of blood and the like that flow through the blood vessels. There is known an imaging method using a time of flight (TOF) effect, a phase contrast (PC) effect or the like for MRA. FBI (Fresh Blood Imaging) has been proposed as an imaging method using no contrast agent (refer to, for example, patent documents 1 and 2).
Patent Document 1. Japanese Unexamined Patent Publication No. 2000-5144.
Patent Document 2. Japanese Unexamined Patent Publication No. 2002-200054.
In the FBI method, an imaging sequence is carried out during cardiac diastole and cardiac systole to produce images about a subject. An MRA image related to the subject is obtained based on the value of a difference between these images. This method applies a flow boid or void of an FSE (Fast Spin Echo) method.
Described specifically, the imaging sequence is carried out during cardiac diastole to produce a first image. For instance, crusher gradient pulses are transmitted in a slice direction without transmitting gradient pulses for flow compensation in a read direction and without transmitting the crusher gradient pulses in a warp direction, thereby carrying out a scan to produce a first image.
The imaging sequence is executed during cardiac systole to produce a second image. For instance, a scan is executed by transmitting crusher gradient pulses in read, warp and slice directions before transmission of read gradient pulses for reading magnetic resonance signals. Thus, flow voids are produced in the respective axial directions to generate a second image.
Thereafter, an MRA image about the subject is obtained based on the value of difference between the first and second images. Since the blood-flow velocity of an artery is fast during cardiac systole here, a signal intensity from the artery becomes low, whereas since the blood-flow velocity of the artery is slow during cardiac diastole, a signal intensity from the artery becomes high. Therefore, the MRA image generated based on the above-described value of difference becomes high in contrast. Described specifically, only a portion in which a flow void has occurred in the second image is projected.
Since, however, the above method encounters difficulties in predicting the degree of occurrence of flow voids, the MRA image might not be produced with sufficient high contrast. It was thus difficult to obtain suitable image quality.
Since no flow void occurs in a flow lying in such a direction that a synthesis in the read and warp directions becomes zero, the flow might not be projected suitably. Therefore, the above method encountered difficulties in producing an MRA image with high accuracy.
Since the MRA image is generated based on the value of difference between the first and second images in the above method, the signal intensity becomes not greater than the first image and noise reaches √{square root over (2)} times. Therefore, there was a case in which obtaining sufficient image quality was difficult because the MRA image became 1/√{square root over (2)} or less in S/N ratio with respect to the first image.
There was a case in which since the acquisition of magnetic resonance signals was limited to the FSE method, general versatility was insufficient in the above method.
Therefore, the above method encountered difficulties in enhancing diagnostic efficiency because general versatility was poor and image quality was deteriorated.